The present invention relates to the measurement of insulation resistance of a rail joint and, more particularly, to an apparatus and method for so measuring insulation resistance in the presence of leakage ground resistance without the use of calibrated meters or the like.
It has been a practice in the art of railroading to employ track circuits as train occupancy detectors, and as communication links to trains and along the right of way for other train signal functions. Insulated joints are a vital element of many types of track circuits. Provided the track circuits have been properly designed, failure of insulated joints does not create an unsafe condition; however, their failure does create nuisance conditions.
It will be understood that an insulated rail joint provides an electrical insulating barrier in the track for the purpose of isolating one track circuit from an adjacent track circuit, or to preclude the propagated rail-carried cab signals from spilling over into an unwanted track section. Also, since the insulated joint does constitute a mechanical joint in the rail, such joints are subjected to high stress levels due to the constant rolling of railroad wheels over them. As a result, insulated joints have a finite life and therefore pose a maintenance problem.
The problem lies in the need for replacement of the insulation in insulated joints, usually entailing replacement of the entire joint, which can be relatively expensive for jointed-rail track and prohibitively expensive for welded rail track. In the case of welded rail, the insulated joint section needs to be cut out and the new insulated joint section has to be welded in its place.
Because of the substantial difficulty in determining the electrical insulating quality of a new insulated joint, especially in the aforenoted welded rail track, it has not been uncommon to remove suspected leaky insulated joints, only to find, after all, that the joint is satisfactory.
Accordingly, there is a need to be able to determine the resistance of an installed insulated joint in a simple, efficient, and reasonably accurate manner. Although at first blush it would appear to be a very simple task, in reality it has not proven to be the case. This might best be appreciated by referring to FIG. 1, which illustrates the problem. As shown therein, it is easy enough to connect an AC or DC signal source such as the source 10 around an insulated joint whose resistance it would be desirable to determine. While it is then a simple task to measure the voltage drop across the insulated joint, as well as the current flowing from the signal source 10, unfortunately only a portion of the current from the signal source flows through the resistance of the insulated joint, that is, resistor R.sub.J. A substantial amount of the source current I2 can flow through the ground on which the track lies, and other run-around paths through the track work, such as impedance bonds and cross bonds.
The present inventors have recognized that the difficulty arises from the fact that there is no easy, direct way of measuring just the current I1 flowing through the insulated joint. One solution would be to measure this current with a clamp-on ammeter around the rail, between the feed point and the insulated joint. Unfortunately, there is no known commercially available clamp-on ammeter with jaws of sufficient size to close around a rail section.
A conventional method which has been widely used for a number of years in the attempt to measure the quality of an insulated joint is shown by reference to FIGS. 2A and 2B. This method is still used by many railroads today. Although this method does not provide a quantitative value for the joint insulation resistance, in many cases it can provide a qualitative measure of the joint resistance.
FIG. 2A illustrates the basic construction of an insulated rail joint. Insulating members 20 provide insulation of the structural metallic splice bars 20A. Members 21 are tubular insulating sleeves which provide insulation of the joint fastening bolts. The "end post" insulating member 22, which is in the shape of the cross-section of the rail, provides the insulation of the rail ends. This end post insulating member is made of such a material that it can stand the high compressive stresses that occur during high ambient temperatures when the rails expand and try to tightly press against each other. All these insulating members work together to electrically insulate one rail from the other. Since high stresses and movement can occur at these mechanical rail joints, this may eventually lead to a mechanical breakdown of the insulating members which cause the insulating quality of the rail joint to deteriorate in time.
FIG. 2B illustrates how the conventional method attempts to determine the quality of an insulated joint. Note that in FIG. 2B, if the resistance of R1 is unacceptably low, but the insulation of the joint or splice bars 20, is good, that is to say, R2, R3, R4, R5 (in FIG. 2B) all have high resistance values, the method will provide results which would indicate that the insulated joint was good, when in reality it was not.